1. Field of the Invention
This invention relates to a focus detecting apparatus suitable for a photographic camera, a video camera or the like. The invention particularly relates to focus detecting apparatus suitable for effecting focus detection with a plurality of distance measuring points or any distance measuring point in a photographing picture plane as a subject. This is achieved the utilization of the so-called image deviation system in which the exit pupil of an objective lens is divided into a plurality of areas and a light beam passing through each of said areas is used to form distributions of quantity of light regarding a plurality of object images and the relative positional relation between these distributions of quantity of light is found to thereby detect the in-focus state of the objective lens.
2. Related Background Art
Light receiving type focus detection systems of relatively high detection accuracy utilizing a light beam passed through an objective lens include a system called the image deviation system.
Among focus detecting apparatuses utilizing this image deviation system, a focus detecting apparatus designed to effect focus detection with respect to the central portion of a photographing range and the other areas is proposed, for example, by Japanese Laid-Open Patent Application 1-120518.
FIGS. 20-22 of the accompanying drawings are schematic views of some portions of the focus detecting apparatus proposed by the above-mentioned publication.
FIG. 20 shows a perspective view, FIG. 21 shows a vertical cross-sectional shape, and FIG. 22 shows the positional relation between the picture element array of a photoelectric conversion device comprising a single chip and the distribution of quantity of light.
The reference numeral 42 designates a multi-opening field mask having juxtaposed rectangular openings each having longer sides in the lateral direction so viewed in FIG. 20, and disposed near the predetermined imaging plane of an objective lens (not shown). The reference numeral 43 denotes a filter for intercepting lights of longer wavelength than near-infrared light, and the reference numeral 50 designates an accumulation type field lens disposed somewhat off the predetermined imaging plane of the objective lens. The accumulation type field lens 50 comprises lens portions 50a, 50b, 50c, 50d, 50e, 50f and 50g differing in optical action from one another as will be described later, and these portions are formed by changing one or both of the lens thickness and the radius of curvature of the lens surface. In lieu of the accumulation type field lens 50, use can be made of a combination of an optical system such as a Fresnel lens comprising different prisms juxtaposed relative to the openings in the multi-opening field mask and a biconvex lens.
The reference numeral 51 denotes a convex lens, and the reference numeral 53 designates a two-image forming lens. These lenses together constitute a re-imaging lens unit with a two-opening stop 52 interposed therebetween, and the convex lens 51 converts incident light into a nearly parallel state (the optical action of this lens is described in Japanese Patent Publication No. 62-33564), and the two-image forming lens 53 comprising two convex lenses 53a and 53b juxtaposed and cemented together forms two secondary images of the object image formed by the objective lens. The aforementioned two-opening stop 52 has vertically long elliptical openings 52a and 52b juxtaposed in the lateral direction as viewed in FIG. 20.
The reference numeral 54 denotes a concave lens for correcting curvature of image field which is disposed on a transparent plastic package 56 containing a photoelectric conversion device 55 (FIGS. 21 and 22) therein. The accumulation type field lens 50, the convex lens 51 of the re-imaging lens unit and the concave lens 54 are shaped vertically long, but all of them are rotation-symmetrical spherical lens systems.
Light beams passed through the openings 42a to 42g in the multi-opening field mask 42 are transmitted through the lens portions 50a, 50b, 50c, 50d, 50e, 50f and 50g, respectively, of the accumulation type field lens 50, as shown in FIG. 20, and form the secondary image of the object image on the photoelectric conversion device 55. FIG. 21 shows this state, and 60a and 60b, . . . , 60m and 60n, and 62a and 62b are sets of picture element arrays (line sensors) each comprising a number of picture elements, and on 62a and 62b, there is formed a filter having a band-pass characteristic substantially equal to the wavelength of emitted light from an object illuminating device which will be described later. The opening images 61a, . . . , 61n of the openings 42a, . . . , 42g in the multi-opening field mask 42 are projected correspondingly to these picture element arrays, and the secondary images of an object are formed therein. At that time, the refractive powers of the optical systems relaying the mask 42 and the device 55, particularly the lens portions of the accumulation type field lens 50 and the re-imaging unit, are adjusted in accordance with the width of each opening in the multi-opening field mask 42 and the width of light intercepting zones 42h, . . . , 42m between the openings, and the width of the picture element arrays on the photoelectric conversion device 55 and the pitch of the picture element arrays and therefore, the light intercapting zones 42h, . . . , 42m of the multi-opening field mask 42 prevent part of a light beam having emerged from a predetermined opening from entering the other picture element arrays than the picture element array (line sensor) corresponding at one to one to said opening. Also, two field mask images are formed in laterally juxtaposed relationship with each other per one opening in the multi-opening field mask 42 by the actions of the openings 52a, 52b and the lens portions 53a, 53b, and in the relation to the position of the object image relative to the predetermined imaging plane, the secondary images of the object therein both move in the direction of arrow A and the direction of arrow B. Accordingly, each set of picture element arrays detects the relative spacing between the distributions of quantity of light regarding secondary images forming a pair on the basis of the photoelectric conversion output and thus, the in-focus state of the objective lens can be known with respect to distance measuring positions at a plurality of points.
In the prior-art focus detecting apparatus designed to effect distance measurement in a plurality of areas, a pair of picture element arrays (line sensors) are required for one distance measuring field. Therefore, an increase in the number of distance measuring fields requires a corresponding increase in the number of picture element arrays, and this has led to the tendency of the chip size of a photoelectric conversion element such as a CCD toward bulkiness. Generally, in the manufacture of a CCD or the like, an increased area thereof means greater technical difficulty.
U.S. application Ser. No. 281,326 (Japanese Laid-Open Patent Application No. 1-154011) discloses a focus detecting apparatus having a plurality of fields.